Equilibrium

  • Many reactions are reversible, meaning they go both ways.

  • To show a reaction is reversible, a double headed arrow, ⇌, is used.

  • e.g. H2(g) + I2(g) ⇌ 2HI(g)

Forwards reaction: H2(g) + I2(g) → 2HI(g)

Backwards reaction: 2HI(g) → H2(g) + I2(g)

  • As the reactants get used up, the forward reaction slows, while more product is formed so the backwards reaction speeds up.

  • At dynamic equilibrium, the rate of the forward reaction will be equal to the rate of the backwards reaction and the concentrations of reactants and product will remain constant.

  • The position of equilibrium indicates how much reactants and how much products there will be at dynamic equilibrium.

  • If the position of equilibrium lies to the left, there will be more reactants.

  • If the position of equilibrium lies to the right, here will be more products.

Equilibrium constant

  • If the concentrations of reactants and products at equilibrium are known (moldm-3), then the equilibrium constant (Kc) can be calculated.

  • Kc will change if the temperature changes.

  • For the general equation: aA + bB ⇌ dD + eE,

Kc = [D]d[E]e / [A]a[B]b

  • The position of equilibrium can be estimated using Kc

  • Kc > 1: the equilibrium lies to the right.

  • Kc >> 1: the equilibrium lies far to the right.

  • Kc < 1: the equilibrium lies to the left.

  • Kc << 1: the equilibrium lies far to the left

  • Changing the concentration of the reactants and/or products will shift the position of equilibrium to keep Kc constant.

  • If the concentration of reactants increases then the position of equilibrium will shift to the right to increase the concentration of products.

  • If the concentration of products is increased then the position of equilibrium will shift to the left to increase the concentration of reactants.

Effect of pressure on the position of equilibrium

  • Increasing pressure will shift the position of equilibrium to the side that has the fewest moles.

  • Decreasing pressure will shift the position of equilibrium to the side that has the most moles.

  • Changing pressure only affects equilibria involving gases

Effect of temperature on the position of equilibrium

  • Increasing temperature shifts the position of equilibrium in the endothermic direction.

  • Decreasing temperature will shift the position of equilibrium in the exothermic direction.